Most of the well known prior art navigation and positioning system, such as LORAN, NAVSTAR, and GPS use special transmitters, either orbiting the earth or land-based, dedicated solely to the positioning system. Great efforts are made in such systems to synchronize the transmitters in these systems so that the phases of the beacon signals reaching a user at any location on the earth's surface can be calculated with a known level of accuracy. These types of prior art systems generally require huge capital investments, often government subsidized, because the transmitters and their control systems are very complex and expensive. In the present invention, the signals used for positioning are the 19 KHz pilot signals transmitted by commercial broadcast stereo FM stations. In the United States, there are nearly 5000 commercial broadcast stereo FM radio stations, and the FCC requires that all commercial stereo FM radio stations broadcast a 19 KHz pilot sub-carrier signal. The maximum allowed deviation is .+-.2 hertz. Generally the rate of frequency drift has been found to be much less than 0.1 hertz per day. In any case, the transmitters used by the present invention are free, which makes the positioning system of the present invention relatively inexpensive to implement.
Two U.S. patents which describe "delta-position" systems using commercial broadcast transmissions are U.S. Pat. Nos. 4,054,880 (Dalabakis et al.) and 3,889,264 (Fletcher). These prior art patents describe systems using three spaced-apart radio signals, each of which is an independent AM radio signal. The systems typically have a vehicle carried mobile receiver, with a separate tuner for each station, and a second receiver at a fixed, known position. Basically, these systems count "zero crossings", each of which indicates that the user has moved a certain distance from his previous location. Thus, if the user needs to know his current position, the user needs to first specify his starting position. A fixed position receiver detects frequency drift of the transmitters, and that drift information is used to adjust and coordinate the zero crossing counts made by the mobile receivers.
These are "delta-position" systems because they determine only the distance and direction travelled by a mobile user from any particular starting point. Neither Dalabakis nor Fletcher actually determines the position of the mobile user, and in fact such a determination is not possible using the Dalabakis or Fletcher systems because these systems do not have the ability to determine the phases of the transmitted radio signals. Furthermore, since only zero crossings are counted, the positioning accomplished by these systems have "granularity", which in the case of the systems disclosed in these two patents is on the order of thirty meters.
Like Dalabakis and Fletcher, the present invention uses spaced apart, commercial radio signals. The present invention also uses a fixed position receiver to help the mobile units determine their position. However, the present invention, unlike Dalabakis and Fletcher, determines the user's position without need for any starting point information, and determines such positions with a high degree of accuracy. To do this, the fixed position receiver not only determines frequency drift, it also determines the relative phases of the various beacons with a very high degree of accuracy (e.g., within about 0.02 degrees, or equivalently, within about 0.00035 radians) using a digital phase-locked loop. Using this relative phase information, the commercial radio signal beacons are transformed, in essence, into coordinated beacon signals with well defined phase relationships. As a result, the position of the mobile users can be computed from the radio signals received by the mobile user, with an accuracy of about .+-.3 feet in any direction.
Two other important differences between the present invention and Dalabakis and Fletcher are (1) the use of 19 KHz beacon sub-carrier signals of commercial broadcast stereo FM stations, and (2) the use of a single receiver for observing the phases of multiple radio signals. FM radio signals are inherently better than the AM radio signals, because FM modulated signals all less susceptible to noise. Further, the hard limiter found at the front of most FM receivers rejects weaker signals, thus providing multipath rejection.
Also important is the use of a single receiver which scans through all the available FM radio station frequencies. This is not simply an economy measure. The key here is that precisely measuring the relative phases of several signals requires that the measurement system not introduce errors into the system--such as the errors that would be caused by having different propagation delays for separate, multiple receiving circuits. A small phase error can produce a relatively large positioning error. By using the same physical circuitry for all signals, the same phase delay through the circuitry occurs for all received signals.